The activity rhythms of the sympatric prawns Palaemon adspersus and Processa edulis from seagrasss meadows of a shallow bay in the Ebro delta (W Mediterranean) were studied in the laboratory using time-lapse video recordings. The experiments were performed in constant conditions of temperature, salinity and darkness, and under artificial light-dark cycles. Freshly collected specimens of both species have an endogenous cycle of activity peaking during the expected night time which persists in constant conditions in the laboratory. Processa edulis was found to bury in the sandy substrate of the aquaria during the expected light period. Strong exogenous responses to artificial cycles of light and darkness were also recorded in the laboratory, the activity being strongly or completely reduced during the light periods, and re-starting suddenly soon after the lights went off.